Less than one percent of individuals with a spinal cord injury (SCI) experience complete functional recovery by hospital discharge. Functional deficits can be improved after discharge by combining rehabilitation with a promising experimental technique known as non-invasive brain stimulation. Non-invasive brain stimulation is believed to further improve recovery by promoting advantageous plasticity across neural substrates that survive in the majority of SCIs. Specifically, we have recently shown that a form of non-invasive brain stimulation, called transcranial direct current stimulation (tDCS), can improve muscle strength up to 15% more than rehabilitation alone by increasing the excitability of pathways to the weaker muscles. The rationale of this proposal is that optimizing the application of tDCS would lead to an even better enhancement of outcomes in those with SCI. My work, similar to others, has shown that in the presence of an SCI, cortical areas representing stronger muscles are larger than those devoted to weaker muscles. Because of such neurophysiological changes, the broad distribution of current from conventional tDCS would not just excite areas devoted to weak muscles but also those of over-represented strong muscles. Here, I test the premise that reducing extraneous excitation of motor cortical areas that represent strong muscles, by using focal tDCS, I would increase corticospinal excitability to pathways devoted to weak muscles and improve their contribution to movement control of the paretic upper extremity compared to conventional tDCS. My preliminary data provides support for my central premise. I have found that individuals receiving the largest current density from conventional tDCS to cortical areas devoted to weak muscles demonstrate the most improvements in hand dexterity. Therefore, my revised application will build off my previous study in SCI to determine if using a more focal form of tDCS can improve movement control of the paretic upper extremity. To test my premise, eighteen subjects with C2-C8 incomplete SCI will receive single sessions of conventional tDCS or focal tDCS, with a new approach called high-definition tDCS (HD-tDCS), during upper-limb movement training in a randomized, sham-controlled crossover experiment. From my cross-over study, I will quantify how focality of tDCS affects corticospinal excitability to pathways devoted to the weak muscles compared to conventional tDCS. Changes in corticospinal excitability will be assessed with transcranial magnetic stimulation. Next, I will measure how focal tDCS affects movement control of the upper extremity in comparison to conventional tDCS. Kinematic movement control will be quantified as change in upper limb movement speed and smoothness of movement. Finally, I will utilize computational modeling to determine the overlap between the tDCS electric field and motor cortical representations devoted to weak and strong muscles. Completion of my CDA-1 will inform what type of tDCS application (focal vs. non- focal) will be most effective at enhancing upper limb function. This is the ideal project for me to expand my engineering background in the field of neuromodulation in SCI. Specifically, with the help of my primary mentor and comprehensive mentoring team, I will begin to be trained in (1) computational modeling and parametrization of direct current (DC) stimulation for SCI, (2) kinematic outcomes for upper limb function in SCI, (3) clinical best practices in SCI and preparation for clinical trial management and (4) aspects to become an independent researcher. My project is highly integrated with my career development plan with the goal of training me to become a leading investigator in clinical neurorehabilitation within the VAMC. My specific training activities, ranging from coursework to physician shadowing, will allow me to develop expertise in technolgies to promote plasticity, circumvent motor pathway damage and ultimately improve functional recovery in patients with neurological injuries or diseases.